ORCID Profile
0000-0002-5449-6131
Current Organisation
Dublin Institute for Advanced Studies
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: American Astronomical Society
Date: 03-2023
Abstract: GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of Φ UL 95 % = 9.7 × 10 − 12 erg cm − 2 s − 1 above E thr = 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.
Publisher: EDP Sciences
Date: 09-2021
DOI: 10.1051/0004-6361/202140962
Abstract: Aims. The identification of PeVatrons, hadronic particle accelerators reaching the knee of the cosmic ray spectrum (few × 10 15 eV), is crucial to understand the origin of cosmic rays in the Galaxy. We provide an update on the unidentified source HESS J1702-420, a promising PeVatron candidate. Methods. We present new observations of HESS J1702-420 made with the High Energy Stereoscopic System (H.E.S.S.), and processed using improved analysis techniques. The analysis configuration was optimized to enhance the collection area at the highest energies. We applied a three-dimensional likelihood analysis to model the source region and adjust non thermal radiative spectral models to the γ -ray data. We also analyzed archival Fermi Large Area Telescope data to constrain the source spectrum at γ -ray energies 10 GeV. Results. We report the detection of γ -rays up to 100 TeV from a specific region of HESS J1702-420, which is well described by a new source component called HESS J1702-420A that was separated from the bulk of TeV emission at a 5.4 σ confidence level. The power law γ -ray spectrum of HESS J1702-420A extends with an index of Γ = 1.53 ± 0.19 stat ± 0.20 sys and without curvature up to the energy band 64−113 TeV, in which it was detected by H.E.S.S. at a 4.0 σ confidence level. This makes HESS J1702-420A a compelling candidate site for the presence of extremely high energy cosmic rays. With a flux above 2 TeV of (2.08 ± 0.49 stat ± 0.62 sys ) × 10 −13 cm −2 s −1 and a radius of (0.06 ± 0.02 stat ± 0.03 sys )°, HESS J1702-420A is outshone – below a few tens of TeV – by the companion HESS J1702-420B. The latter has a steep spectral index of Γ = 2.62 ± 0.10 stat ± 0.20 sys and an elongated shape, and it accounts for most of the low-energy HESS J1702-420 flux. Simple hadronic and leptonic emission models can be well adjusted to the spectra of both components. Remarkably, in a hadronic scenario, the cut-off energy of the particle distribution powering HESS J1702-420A is found to be higher than 0.5 PeV at a 95% confidence level. Conclusions. For the first time, H.E.S.S. resolved two components with significantly different morphologies and spectral indices, both detected at 5 σ confidence level, whose combined emissions result in the source HESS J1702-420. We detected HESS J1702-420A at a 4.0 σ confidence level in the energy band 64−113 TeV, which brings evidence for the source emission up to 100 TeV. In a hadronic emission scenario, the hard γ -ray spectrum of HESS J1702-420A implies that the source likely harbors PeV protons, thus becoming one of the most solid PeVatron candidates detected so far in H.E.S.S. data. However, a leptonic origin of the observed TeV emission cannot be ruled out either.
Publisher: Springer Science and Business Media LLC
Date: 20-11-2019
DOI: 10.1038/S41586-019-1743-9
Abstract: Gamma-ray bursts (GRBs) are brief flashes of γ-rays and are considered to be the most energetic explosive phenomena in the Universe
Publisher: Oxford University Press (OUP)
Date: 16-06-2022
Abstract: We report on a search for persistent radio emission from the one-off fast radio burst (FRB) 20190714A, as well as from two repeating FRBs, 20190711A and 20171019A, using the MeerKAT radio telescope. For FRB 20171019A, we also conducted simultaneous observations with the High-Energy Stereoscopic System (H.E.S.S.) in very high-energy gamma rays and searched for signals in the ultraviolet, optical, and X-ray bands. For this FRB, we obtain a UV flux upper limit of $1.39 \\times 10^{-16}~{\\rm erg\\, cm^{-2}\\, s^{-1}}$Å−1, X-ray limit of $\\sim 6.6 \\times 10^{-14}~{\\rm erg\\, cm^{-2}\\, s^{-1}}$ and a limit on the very high energy gamma-ray flux $\\Phi (E\\gt 120\\, {\\rm GeV}) \\lt 1.7\\times 10^{-12}\\, \\mathrm{erg\\, cm^{-2}\\, s^{-1}}$. We obtain a radio upper limit of ∼15 $\\mu$Jy beam−1 for persistent emission at the locations of both FRBs 20190711A and 20171019A with MeerKAT. However, we detected an almost unresolved (ratio of integrated flux to peak flux is ∼1.7 beam) radio emission, where the synthesized beam size was ∼ 8 arcsec size with a peak brightness of $\\sim 53\\, \\mu$Jy beam−1 at MeerKAT and $\\sim 86\\, \\mu$Jy beam−1 at e-MERLIN, possibly associated with FRB 20190714A at z = 0.2365. This represents the first detection of persistent continuum radio emission potentially associated with a (as-yet) non-repeating FRB. If the association is confirmed, one of the strongest remaining distinction between repeaters and non-repeaters would no longer be applicable. A parallel search for repeat bursts from these FRBs revealed no new detections down to a fluence of 0.08 Jy ms for a 1 ms duration burst.
Publisher: EDP Sciences
Date: 12-2020
DOI: 10.1051/0004-6361/202038851
Abstract: The unidentified very-high-energy (VHE E 0.1 TeV) γ -ray source, HESS J1826−130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady γ -ray flux from HESS J1826−130, which appears extended with a half-width of 0.21° ± 0.02 stat ° ± 0.05 sys °. The source spectrum is best fit with either a power-law function with a spectral index Γ = 1.78 ± 0.10 stat ± 0.20 sys and an exponential cut-off at 15.2 −3.2 +5.5 TeV, or a broken power-law with Γ 1 = 1.96 ± 0.06 stat ± 0.20 sys , Γ 2 = 3.59 ± 0.69 stat ± 0.20 sys for energies below and above E br = 11.2 ± 2.7 TeV, respectively. The VHE flux from HESS J1826−130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula, HESS J1825−137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826−130 VHE emission related to PSR J1826−1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826−130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to ≳200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants, molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.
Publisher: Sissa Medialab
Date: 07-07-2021
DOI: 10.22323/1.395.0809
Publisher: EDP Sciences
Date: 06-2019
DOI: 10.1051/0004-6361/201935242
Abstract: Young core-collapse supernovae with dense-wind progenitors may be able to accelerate cosmic-ray hadrons beyond the knee of the cosmic-ray spectrum, and this may result in measurable gamma-ray emission. We searched for gamma-ray emission from ten supernovae observed with the High Energy Stereoscopic System (H.E.S.S.) within a year of the supernova event. Nine supernovae were observed serendipitously in the H.E.S.S. data collected between December 2003 and December 2014, with exposure times ranging from 1.4 to 53 h. In addition we observed SN 2016adj as a target of opportunity in February 2016 for 13 h. No significant gamma-ray emission has been detected for any of the objects, and upper limits on the TeV gamma-ray flux of the order of ~10 −13 cm −2 s −1 are established, corresponding to upper limits on the luminosities in the range ~2 × 10 39 to ~1 × 10 42 erg s −1 . These values are used to place model-dependent constraints on the mass-loss rates of the progenitor stars, implying upper limits between ~2 × 10 −5 and ~2 × 10 −3 M ⊙ yr −1 under reasonable assumptions on the particle acceleration parameters.
Publisher: Springer Science and Business Media LLC
Date: 17-06-2020
Publisher: American Astronomical Society
Date: 06-2023
Abstract: Magnetic fields in galaxies and galaxy clusters are believed to be the result of the lification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron−positron pairs, which can subsequently initiate electromagnetic cascades. The gamma-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of B 7.1 × 10 −16 G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of 10 4 (10 7 ) yr, IGMF strengths below 1.8 × 10 −14 G (3.9 × 10 −14 G) are excluded, which rules out specific models for IGMF generation in the early universe.
Publisher: EDP Sciences
Date: 18-11-2015
Publisher: EDP Sciences
Date: 06-2022
DOI: 10.1051/0004-6361/202243096
Abstract: Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 yr by establishing Cassiopeia A and the remnant of Tycho’s SN as very-high-energy (VHE) γ -ray sources. The remnant of Kepler’s SN, which is the product of the most recent naked-eye SN in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the γ -ray luminosities of the remnants of Tycho’s and Kepler’s SNe are similar, then the latter is expected to be one of the faintest γ -ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6 σ for a VHE signal from the remnant of Kepler’s SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 h. The measured integral flux above an energy of 226 GeV is ∼0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a γ -ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi -LAT. The overall SED is similar to that of the remnant of Tycho’s SN, possibly indicating the same nonthermal emission processes acting in both these young remnants of thermonuclear SNe.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 04-2022
Abstract: Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material slams into the companion star’s wind can accelerate particles. We report very-high-energy (VHE ≳ 100 giga–electron volts ) gamma rays from the recurrent nova RS Ophiuchi, up to 1 month after its 2021 outburst, observed using the High Energy Stereoscopic System (H.E.S.S.). The temporal profile of VHE emission is similar to that of lower-energy giga–electron volt emission, indicating a common origin, with a 2-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic rays to very high energies.
Publisher: EDP Sciences
Date: 05-2023
Publisher: EDP Sciences
Date: 10-2022
DOI: 10.1051/0004-6361/202244323
Abstract: Context. Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy γ -ray source HESS J1646−458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. Aims. We aim to identify the physical processes responsible for the γ -ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better. Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the γ -ray emission of HESS J1646−458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. Results. We detected large-scale (∼2° diameter) γ -ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with γ -ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the γ -ray emission with gas clouds as identified through H I and CO observations. Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the γ -ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.
Publisher: EDP Sciences
Date: 09-2022
DOI: 10.1051/0004-6361/202243531
Abstract: The nearby, massive, runaway star ζ Ophiuchi has a large bow shock detected in optical and infrared light and, uniquely among runaway O stars, diffuse X-ray emission detected from the shocked stellar wind. Here we make the first detailed computational investigation of the bow shock of ζ Ophiuchi, to test whether a simple model of the bow shock can explain the observed nebula, and to compare the detected X-ray emission with simulated emission maps. We reanalysed archival Chandra observations of the thermal diffuse X-ray emission from the shocked wind region of the bow shock, finding total unabsorbed X-ray flux in the 0.3–2keV band corresponding to a diffuse X-ray luminosity of L X = 2.33 −1.54 +1.12 × 10 29 erg s −1 , consistent with previous work. The diffuse X-ray emission arises from the region between the star and the bow shock. Three-dimensional magnetohydrodyanmic simulations were used to model the interaction of the star’s wind with a uniform interstellar medium (ISM) using a range of stellar and ISM parameters motivated by observational constraints. Synthetic infrared, Hα, soft X-ray, emission measure, and radio 6 GHz emission maps were generated from three simulations, for comparison with the relevant observations. Simulations where the space velocity of ζ Ophiuchi has a significant radial velocity produce infrared emission maps with the opening angle of the bow shock in better agreement with observations than for the case where motion is fully in the plane of the sky. All three simulations presented here have X-ray emission fainter than observed, in contrast to results for NGC 7635. The simulation with the highest pressure has the closest match to X-ray observations, with a flux level within a factor of two of the observational lower limit, and emission weighted temperature of log 10 ( T A /K) = 6.4, although the morphology of the diffuse emission appears somewhat different. The observed X-ray emission is from a filled bubble that is brightest near the star, whereas simulations predict brightening towards the contact discontinuity as density increases. This first numerical study of the bow shock and wind bubble around ζ Ophiuchi uses a relatively simple model of a uniform ISM and ideal-magnetohydrodynamics, and can be used as a basis for comparing results from models incorporating more physical processes, or higher resolution simulations that may show more turbulent mixing.
Publisher: EDP Sciences
Date: 07-2019
DOI: 10.1051/0004-6361/201935704
Abstract: The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100 MeV E 100 GeV) γ -ray band, which is continuously monitored with Fermi -LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E 100 GeV) γ -ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8.7 σ significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 ± 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r ≳ 1.7 × 10 17 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279 .
Publisher: EDP Sciences
Date: 04-2023
Publisher: American Association for the Advancement of Science (AAAS)
Date: 04-06-2021
Publisher: EDP Sciences
Date: 2020
DOI: 10.1051/0004-6361/201935906
Abstract: Context. Flat-spectrum radio-quasars (FSRQs) are rarely detected at very high energies ( E ≥ 100 GeV) due to their low-frequency-peaked spectral energy distributions. At present, only six FSRQs are known to emit very high-energy (VHE) photons, representing only 7% of the VHE extragalactic catalog, which is largely dominated by high-frequency-peaked BL Lacertae objects. Aims. Following the detection of MeV–GeV γ -ray flaring activity from the FSRQ PKS 0736+017 ( z = 0.189) with Fermi -LAT, the H.E.S.S. array of Cherenkov telescopes triggered target-of-opportunity (ToO) observations on February 18, 2015, with the goal of studying the γ -ray emission in the VHE band. Methods. H.E.S.S. ToO observations were carried out during the nights of February 18, 19, 21, and 24, 2015. Together with Fermi -LAT, the multi-wavelength coverage of the flare includes Swift observations in soft X-ray and optical-UV bands, and optical monitoring (photometry and spectro-polarimetry) by the Steward Observatory, and the ATOM, the KAIT, and the ASAS-SN telescopes. Results. VHE emission from PKS 0736+017 was detected with H.E.S.S. only during the night of February 19, 2015. Fermi -LAT data indicate the presence of a γ -ray flare, peaking at the time of the H.E.S.S. detection, with a flux doubling timescale of around six hours. The γ -ray flare was accompanied by at least a 1 mag brightening of the non-thermal optical continuum. No simultaneous observations at longer wavelengths are available for the night of the H.E.S.S. detection. The γ -ray observations with H.E.S.S. and Fermi -LAT are used to put constraints on the location of the γ -ray emitting region during the flare: it is constrained to be just outside the radius of the broad-line region r B L R with a bulk Lorentz factor Γ ≃ 20, or at the level of the radius of the dusty torus r torus with Γ ≃ 60. Conclusions. PKS 0736+017 is the seventh FSRQ known to emit VHE photons, and at z = 0.189 is the nearest so far. The location of the γ -ray emitting region during the flare can be tightly constrained thanks to opacity, variability, and collimation arguments.
Publisher: Springer Science and Business Media LLC
Date: 10-08-2014
DOI: 10.1038/NATURE13522
Abstract: Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings. This picture has been challenged by the discovery of a dense and almost static shell that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse's wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind and forms an almost static, photoionization-confined shell. Other red supergiants should have much more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova light curve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.
Publisher: EDP Sciences
Date: 04-2021
DOI: 10.1051/0004-6361/202038949
Abstract: The flat spectrum radio quasar (FSRQ) PKS 1510−089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E 100 GeV) γ rays. The VHE γ -ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE γ -ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE γ -ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R -band observations with ATOM revealed a counterpart of the γ -ray flare, even though the detailed flux evolution differs from the VHE γ -ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ -ray flare. In the high-energy (HE, E 100 MeV) γ -ray band, only a moderate flux increase was observed with Fermi -LAT, while the HE γ -ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ -ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.
Publisher: American Astronomical Society
Date: 23-08-2023
Abstract: We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2.°2 away from the best-fit position of the IceCube neutrino event IceCube-211208A detected on 2021 December 8. The source was in a high-flux state in the optical, ultraviolet, X-ray, and GeV γ -ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the γ -ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and γ -ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed γ -ray spectral cutoff in both the leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.
Publisher: EDP Sciences
Date: 28-10-2021
DOI: 10.1051/0004-6361/202141486
Abstract: Context. Supernova remnants (SNRs) are commonly thought to be the dominant sources of Galactic cosmic rays up to the knee of the cosmic-ray spectrum at a few PeV. Imaging Atmospheric Cherenkov Telescopes have revealed young SNRs as very-high-energy (VHE, GeV) gamma-ray sources, but for only a few SNRs the hadronic cosmic-ray origin of their gamma-ray emission is indisputably established. In all these cases, the gamma-ray spectra exhibit a spectral cutoff at energies much below 100 TeV and thus do not reach the PeVatron regime. Aims. The aim of this work was to achieve a firm detection for the oxygen-rich SNR LMC N132D in the VHE gamma-ray domain with an extended set of data, and to clarify the spectral characteristics and the localization of the gamma-ray emission from this exceptionally powerful gamma-ray-emitting SNR. Methods. We analyzed 252 h of High Energy Stereoscopic System (H.E.S.S.) observations towards SNR N132D that were accumulated between December 2004 and March 2016 during a deep survey of the Large Magellanic Cloud, adding 104 h of observations to the previously published data set to ensure a 5 σ detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi -LAT Pass 8 data was also included. Results. We unambiguously detect N132D at VHE with a significance of 5.7 σ . We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi -LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. Conclusions. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray emission is best explained by a dominant hadronic component formed by diffusive shock acceleration. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position.
Publisher: Oxford University Press (OUP)
Date: 19-03-2021
Abstract: Expanding nebulae are produced by mass-loss from stars, especially during late stages of evolution. Multidimensional simulation of these nebulae requires high resolution near the star and permits resolution that decreases with distance from the star, ideally with adaptive time-steps. We report the implementation and testing of static mesh-refinement in the radiation-magnetohydrodynamics (R-MHD) code pion, and document its performance for 2D and 3D calculations. The bow shock produced by a hot, magnetized, slowly rotating star as it moves through the magnetized ISM is simulated in 3D, highlighting differences compared with 2D calculations. Latitude-dependent, time-varying magnetized winds are modelled and compared with simulations of ring nebulae around blue supergiants from the literature. A 3D simulation of the expansion of a fast wind from a Wolf–Rayet star into the slow wind from a previous red supergiant phase of evolution is presented, with results compared with results in the literature and analytic theory. Finally, the wind–wind collision from a binary star system is modelled with 3D MHD, and the results compared with previous 2D hydrodynamic calculations. A python library is provided for reading and plotting simulation snapshots, and the generation of synthetic infrared emission maps using torus is also demonstrated. It is shown that state-of-the-art 3D MHD simulations of wind-driven nebulae can be performed using pion with reasonable computational resources. The source code and user documentation is made available for the community under a BSD3 licence.
Publisher: Oxford University Press (OUP)
Date: 17-12-2021
Abstract: G0.253+0.016, commonly referred to as ‘the Brick’ and located within the Central Molecular Zone, is one of the densest (≈103–4 cm−3) molecular clouds in the Galaxy to lack signatures of widespread star formation. We set out to constrain the origins of an arc-shaped molecular line emission feature located within the cloud. We determine that the arc, centred on $\\lbrace l_{0},b_{0}\\rbrace =\\lbrace 0{_{.}^{\\circ}} 248,\\, 0{_{.}^{\\circ}} 018\\rbrace$, has a radius of 1.3 pc and kinematics indicative of the presence of a shell expanding at $5.2^{+2.7}_{-1.9}$ $\\mathrm{\\, km\\, s}^{-1}$. Extended radio continuum emission fills the arc cavity and recombination line emission peaks at a similar velocity to the arc, implying that the molecular gas and ionized gas are physically related. The inferred Lyman continuum photon rate is NLyC = 1046.0–1047.9 photons s−1, consistent with a star of spectral type B1-O8.5, corresponding to a mass of ≈12–20 M⊙. We explore two scenarios for the origin of the arc: (i) a partial shell swept up by the wind of an interloper high-mass star and (ii) a partial shell swept up by stellar feedback resulting from in situ star formation. We favour the latter scenario, finding reasonable (factor of a few) agreement between its morphology, dynamics, and energetics and those predicted for an expanding bubble driven by the wind from a high-mass star. The immediate implication is that G0.253+0.016 may not be as quiescent as is commonly accepted. We speculate that the cloud may have produced a ≲103 M⊙ star cluster ≳0.4 Myr ago, and demonstrate that the high-extinction and stellar crowding observed towards G0.253+0.016 may help to obscure such a star cluster from detection.
Publisher: EDP Sciences
Date: 03-2020
DOI: 10.1051/0004-6361/201936761
Abstract: Aims. Colliding wind binary systems have long been suspected to be high-energy (HE 100 MeV E 100 GeV) γ -ray emitters. η Car is the most prominent member of this object class and is confirmed to emit phase-locked HE γ rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE E GeV) γ -ray emission from η Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.). Methods. The region around η Car was observed with H.E.S.S. between orbital phase p = 0.78−1.10, with a closer s ling at p ≈ 0.95 and p ≈ 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the η Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions. Results. H.E.S.S. detected VHE γ -ray emission from the direction of η Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE γ rays agree within statistical and systematic errors before and after periastron. The γ -ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.
Publisher: IOP Publishing
Date: 09-2020
DOI: 10.1088/1742-6596/1620/1/012012
Abstract: Initial results are presented from 3D MHD modelling of stellar-wind bubbles around O stars moving supersonically through the ISM. We describe algorithm updates that enable high-resolution 3D MHD simulations at reasonable computational cost. We apply the methods to the simulation of the astrosphere of a rotating massive star moving with 30km s −1 through the diffuse interstellar medium, for two different stellar magnetic field strengths, 10G and 100G. Features in the flow are described and compared with similar models for the Heliosphere. The shocked interstellar medium becomes asymmetric with the inclusion of a magnetic field misaligned with the star’s direction of motion, with observable consequences. When the Alfv´enic Mach number of the wind is ≤ 10 then the stellar magnetic field begins to affect the structure of the wind bubble and features related to the magnetic axis of the star become visible at parsec scales. Prospects for predicting and measuring non-thermal radiation are discussed.
Publisher: American Astronomical Society
Date: 12-2021
Abstract: We report on the observations of four well-localized binary black hole (BBH) mergers by the High Energy Stereoscopic System (H.E.S.S.) during the second and third observing runs of Advanced LIGO and Advanced Virgo, O2 and O3. H.E.S.S. can observe 20 deg 2 of the sky at a time and follows up gravitational-wave (GW) events by “tiling” localization regions to maximize the covered localization probability. During O2 and O3, H.E.S.S. observed large portions of the localization regions, between 35% and 75%, for four BBH mergers (GW170814, GW190512_180714, GW190728_064510, and S200224ca). For these four GW events, we find no significant signal from a pointlike source in any of the observations, and we set upper limits on the very high energy ( GeV) γ -ray emission. The 1–10 TeV isotropic luminosity of these GW events is below 10 45 erg s −1 at the times of the H.E.S.S. observations, around the level of the low-luminosity GRB 190829A. Assuming no changes are made to how follow-up observations are conducted, H.E.S.S. can expect to observe over 60 GW events per year in the fourth GW observing run, O4, of which eight would be observable with minimal latency.
Publisher: American Astronomical Society
Date: 07-09-2023
Start Date: 2019
End Date: End date not available
Funder: Irish Research Council
View Funded Activity